Theoretical study of multiple high-order harmonic generation by intense ultrashort pulsed laser fields: A new generalized pseudospectral time-dependent method

We present a new time-dependent method for accurate and efficient non-perturbative treatment of multiple high-order harmonic generation (HHG) in intense laser fields. The time development of the wavefunction is obtained by a new split-operator time-propagation technique in the energy representation. The generalized pseudospectral technique is extended to perform an optimal spatial grid discretization, leading to significant improvement of the quality of the wavefunction over that obtained by the equal-spacing spatial discretization techniques. The accuracy of the method is demonstrated by several benchmark calculations including the excellent agreement of the HHG power spectra in length and acceleration forms. The method is applied to a detailed study of the coherent control of HHG spectra of atomic hydrogen in intense ultrashort pulsed laser fields with or without the chirp. Several novel chirp-dependent and pulse-duration-dependent phenomena are predicted. Comparison with available experimental observation is made. (C) 1997 Elsevier Science B.V.